A Keggin-type polyoxotungstate-coordinated diplatinum(II) complex: Synthesis, characterization, and stability of the cis-platinum(II) moieties in dimethylsulfoxide and water

Article abstract of DOI:10.1016/j.inoche.2011.03.049

The synthesis of a Keggin-type polyoxotungstate-coordinated diplatinum(II) complex, [(CH₃)₄N]₃[α-PW ₁₁O₃₉{cis-Pt(NH₃)₂}₂], obtained by reaction of Keggin-type mono-lacunary polyoxotungstate, [α-PW₁₁O₃₉]^7-, with cis- diamminedichloroplatinum(II) in an aqueous solution is described. The complex was characterized by elemental analysis, thermogravimetric/differential thermal analysis (TG/DTA), Fourier transform infrared (FTIR), ultraviolet-visible (UV-vis), and solution ¹H, ³¹P, and ¹⁸³W nuclear magnetic resonance (NMR) spectroscopy. The two cis-platinum(II) moieties, [cis-Pt(NH₃)₂]²⁺, were coordinated each to two oxygen atoms in a mono-vacant site of [α-PW₁₁O ₃₉]^7- with C_s symmetry, and the cis-conformation was highly stable in dimethylsulfoxide and water.

Full text of DOI:10.1016/j.inoche.2011.03.049

Inorganic Chemistry Communications 14 (2011) 982–985
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
A Keggin-type polyoxotungstate-coordinated diplatinum(II) complex: Synthesis,
characterization, and stability of the cis-platinum(II) moieties in dimethylsulfoxide
and water
⁎
Masao Kato, Chika Nozaki Kato
Department of Chemistry, Shizuoka University, Ohya, Suruga-ku, Shizuoka 422-8529, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis of
PW11O39{cis-Pt(NH3)2}2], obtained by reaction of Keggin-type mono-lacunary polyoxotungstate, [α-
PW11O39
7−, with cis-diamminedichloroplatinum(II) in an aqueous solution is described. The complex was
a Keggin-type polyoxotungstate-coordinated diplatinum(II) complex, [(CH3)4N]3[α-
Received 16 February 2011
Accepted 25 March 2011
Available online 2 April 2011
]
characterized by elemental analysis, thermogravimetric/differential thermal analysis (TG/DTA), Fourier
transform infrared (FTIR), ultraviolet-visible (UV–vis), and solution 1H, 31P, and 183W nuclear magnetic
Keywords:
resonance (NMR) spectroscopy. The two cis-platinum(II) moieties, [cis-Pt(NH3)2]2+, were coordinated each
Polyoxometalate
cis-platinum(II) moiety
Nuclear magnetic resonance
Stability
7−
to two oxygen atoms in a mono-vacant site of [α-PW11O39
]
with Cs symmetry, and the cis-conformation
was highly stable in dimethylsulfoxide and water.
© 2011 Elsevier B.V. All rights reserved.
Introduction
(Me4N-1) (Fig. 1), using cisplatin (cis-diamminedichloroplatinum(II)), a
well-known antitumor compound [1]. Furthermore, the stability of
the two platinum(II) moieties with cis-configuration in polyoxoanion 1
was investigated in dimethylsulfoxide (DMSO) and aqueous media.
Platinum and its derivatives, e.g., alloys, oxides, organometallics,
and inorganic compounds, have attracted considerable attention due
to their efficient physical and chemical properties [1]. Since platinum
is an imperative metal for catalysts, electronics, and pharmaceutical
compounds, numerous studies on the syntheses, properties, and
activities of various platinum compounds have been reported; however,
there is still a need for improved properties and activities.
Experimental
Materials and methods
Polyoxometalates (POMs) are of particular interest in the fields of
catalysis, surface science, and materials science because their
chemical properties, such as redox potentials, acidities, and solubility
in various media, can be finely tuned by choosing appropriate
constituent elements and countercations [2]. In particular, the
coordination of various species, e.g., metal ions and organometallics,
into the vacant site(s) of lacunary polyoxometalates is one of the most
powerful techniques used for construction and stabilization of
efficient and well-defined metal centers. However, only a few
examples regarding platinum species-coordinated POMs, e.g., K12
{[WZnPtII2(H2O)2](ZnW9O34)2}·36H2O [3], K7Na9[O=PtIV(H2O)
(PW9O34)2]⋅21.5H2O [4], Na5[H2PtIVV9O28]⋅21H2O [5], and (NBu4)7
[{Pt(cod)}(P2W15V3O62)]⋅2NBu4BF4 (cod=1,5-cyclooctadiene) [6],
have been reported, and some of the results are still debated [7].
In this paper, we report the full details of the synthesis and
characterization of the tetramethylammonium salt of a diplatinum(II)-
coordinated polyoxoanion, [(CH3)4N]3[PW11O39{cis-Pt(NH3)2}2]·10H2O
K7[PW11O39]⋅13H2O was prepared as described in the literature
[8]. The number of solvated water molecules was determined by
thermogravimetric/differential thermal analysis (TG/DTA). All re-
agents and solvents were obtained and used as received from
commercial sources. The elemental analysis was carried out by
Mikroanalytisches Labor Pascher (Remagen, Germany). The sample
was dried overnight at room temperature under 10−3–10−4 torr
before analysis. Infrared spectra were recorded on a Perkin–Elmer
Spectrum100 Fourier transform infrared (FTIR) spectrometer using
KBr disks at room temperature. TG and DTA data were obtained using
a Rigaku Thermo Plus 2 series TG/DTA TG 8120. TG/DTA measure-
ments were performed in air with a temperature increase of 4 °C per
min between 20 and 500 °C. The 1H (600.17 MHz) and 31P-{1H}
(242.95 MHz) nuclear magnetic resonance (NMR) spectra in solutions
were recorded in 5-mm outer diameter tubes on a JEOL ECA-600 NMR
spectrometer (Shizuoka University). 1H NMR spectra were measured
in DMSO-d6 with reference to an internal 3-(trimethylsilyl)-1-
propanesulfonic acid, sodium salt (DSS). Chemical shifts are reported
as positive for resonances downfield of DSS (δ 0). The 31P NMR spectra
were measured in DMSO-d6 and D2O with reference to an external
standard of 85% H3PO4 in a sealed capillary. Chemical shifts were
⁎
Corresponding author. Tel.: +81 54 2384764; fax: +81 54 2373384.
E-mail address: sckatou@ipc.shizuoka.ac.jp (C.N. Kato).
1387-7003/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2011.03.049

M. Kato, C.N. Kato / Inorganic Chemistry Communications 14 (2011) 982–985
983
region) showed: 1101w, 1046w, 953vs, 917 m, 895 m, 861 s, 805vs,
753 s, 721 s, 595w, and 513w cm−1. NMR results gave 1H NMR
(DMSO-d6, 23.7 °C): 2.48–2.50 (DMSO), 3.13 (CH3), 3.32 (H2O), 4.18
(NH3), and 4.32 (NH3); 31P NMR: (DMSO-d6, 23.8 °C): δ −12.85; 183
W
NMR (DMSO-d6, 21.8 °C): δ −101 (2 W), −102 (2 W), −107 (1 W),
−116 (2 W), −142 (2 W), and −162 (2 W). UV–vis absorption (in
H2O, 7.2×10−5 M) showed: λ 251 nm (ε 40,120 M−1cm−1), λ 324 nm
(ε 6787 M−1cm−1), and 405 nm (ε 1346 M−1cm−1).
Results and discussion
Synthesis and spectral characterization of Me4N-1
The polyoxoanion, [PW11O39{cis-Pt(NH3)2}2]3− (1), was formed
by a 2:1 stochiometric reaction of cisplatin with mono-lacunary
Keggin POM, [α-PW11O39]
7−, in an aqueous solution at 25 °C under
air, followed by reprecipitation from water. Faster formation of 1 was
observed when excess cisplatin was used. During the reaction of
7−
cisplatin with [α-PW11O39
]
in aqueous solution, cationic aqua-
platinum complexes, e.g., [cis-Pt(NH3)2Cl(H2O)]+ and [cis-Pt(NH3)2
(H2O)2]2+, were initially formed by elimination of chloride ions [9].
Next, the two cationic aqua-platinum species coordinated to two
Fig. 1. Polyhedral representation of [PW11O39{cis-Pt(NH3)2}2]3− with Cs symmetry. The
WO6 and PO4 sites are represented by white octahedra and gray tetrahedra,
respectively.
oxygen atoms of the mono-lacunary site in [α-PW11O39]
7−. The
reported as negative on the δ scale for resonance upfield of H3PO4
(δ 0). The 183W NMR (25.00 MHz) spectrum was recorded in a tube
(outer diameter: 10 mm) on a JEOL ECA-600 NMR spectrometer
(Kyusyu University). The 183W NMR spectrum measured in DMSO-d6
was referenced to an external standard of saturated Na2WO4-D2O
solution (substitution method). Chemical shifts were reported as
negative for the resonance upfield of Na2WO4 (δ 0). Solution
ultraviolet-visible (UV–vis) spectra were recorded on a Perkin–
Elmer Spectrum Lambda 650 spectrophotometer.
compound Me4N-1 was finally isolated as an analytically pure, yellow
powder in a 14% yield. A control experiment was carried by
suspending transplatin (trans-diamminedichloroplatinum(II))
(0.060 g; 0.2 mmol) in 75 mL of water (at this stage, only a small
amount of transplatin was soluble in water). The suspension was
added to an aqueous solution of K7[α-PW11O39]·13H2O (0.32 g;
0.1 mmol) in 15 mL of water. After stirring for 24 h at 25 °C, solid
(CH3)4NBr (1.546 g; 10 mmol) was added and stirred overnight. Then,
a yellow precipitate was collected by a membrane filter (JG 0.2 μm).
The 31P NMR spectrum of the obtained product in DMSO-d6 showed
no signal, indicating that the polyoxometalate-coordinated platinum
complex was not obtained using transplatin as a starting material
under the present conditions.
The sample was dried overnight at room temperature under a
vacuum of 10−3–10−4 torr for elemental analysis. The elemental results
for C, H, N, Pt, P, and W were in good agreement with the calculated
values for the chemical formula of Me4N-1 with three hydrated water
molecules (see Experimental). It should be noted that Cl analysis
showed no contamination of unreacted cisplatin. The weight loss
observed during drying before analysis was 3.73% for Me4N-1,
corresponding to seven weakly solvated or adsorbed water molecules.
On the other hand, during TG/DTA under atmospheric conditions, a
weight loss of 5.24% was observed below 250 °C, corresponding to 10
water molecules. Therefore, the number of water molecules (10) ob-
served by TG/DTA under atmospheric conditions was consistent
with the sum of the hydrated water molecules (3) indicated by
elemental analysis and the number of water molecules (7) correspond-
ing to the weight loss observed during drying before analysis. In
addition, a weight loss of 8.28% was observed in the temperature range
from 250 to 370 °C, corresponding to the sum of three tetramethy-
lammonium ions and four ammonia molecules.
[(CH3)4N]3[PW11O39{cis-Pt(NH3)2}2]·10H2O (Me4N-1)
A solution of cis-Pt(NH3)2Cl2 (0.060 g; 0.2 mmol) dissolved in
75 mL of water was added to a solution of K7[PW11O39]·13H2O
(0.319 g; 0.1 mmol) dissolved in 10 mL of water. After stirring for 24 h
at 25 °C, solid (CH3)4NBr (1.535 g; 10 mmol) was added and stirred
for 24 h. Then, a yellow precipitate was collected by a membrane filter
(JG 0.2 μm). At this stage, a crude product was obtained in a 0.251 g
yield. For purification, the crude product (0.241 g) was dissolved in
17.0 mL of H2O at 60 °C, followed by standing in a refrigerator
overnight. A yellow precipitate was collected by a membrane filter (JG
0.2 μm) and washed with ethanol. The obtained product was 0.050 g
(the yield calculated on the basis of [mol of Me4N-1]/[mol of K7
[PW11O39]·13H2O]×100 was 14%). When the crude product (0.590 g)
obtained by three cycles of preparation was dissolved in 36 mL of H2O
at 60 °C, the yield was 23% (0.246 g). The obtained product was
soluble in water and DMSO and insoluble in methanol, ethanol,
acetone, and diethyl ether. Elemental analysis results showed: C, 4.32;
H, 1.54; N, 2.92; P, 0.90; W, 59.1; Pt, 11.5; Cl, b0.02; K, b0.015%.
Calculations for [(CH3)4N]3[PW11O39{cis-Pt(NH3)2}2]·xH2O (x=3)=
H
54C12N7Pt2O42P1W11: C, 4.22; H, 1.60; N, 2.87; P, 0.91; W, 59.27; Pt,
11.44%; Cl, 0%; K, 0%. A weight loss of 3.73% was observed during
overnight drying at room temperature under 10−3–10−4 torr before
analysis, suggesting the presence of seven weakly solvated or
adsorbed water molecules (3.56%). TG/DTA under atmospheric
conditions showed a weight loss of 5.24% with endothermic points
at 37 °C observed below 250 °C; calculations showed 5.09% for 10
water molecules. Additionally, a weight loss of 8.28% with two
exothermic peaks at 290 and 317 °C was observed in the temperature
range from 250 to 370 °C; calculations showed three [(CH3)4N]+
(calcd. 6.29%) and four NH3 (calcd. 1.93%) molecules (total: 8.22%). IR
(KBr disk) results in the 1300–400 cm−1 region (polyoxometalate
The FTIR spectrum of Me4N-1 measured as a KBr disk is shown in
Fig. S1. The spectral pattern of Me4N-1 (1101, 1046, 953, 917, 895,
861, 805, 753, and 721 cm−1) was different from that of K7[α-
PW11O39]⋅13H2O (1086, 1043, 953, 903, 862, 810, and 734 cm−1) and
that of Na3[α-PW12O40]⋅12H2O [10] (1080, 984, 893, and 808 cm−1),
suggesting that the two platinum(II) moieties were coordinated to the
7−
vacant site of [α-PW11O39
]
.
The 31P NMR spectrum of Me4N-1 in DMSO-d6 at ~25 °C showed a
clear single-line spectrum at −12.85 ppm due to the internal
phosphorus atom, confirming the purity and homogeneity of the
sample (Fig. S2). The signal was shifted compared with the signals of

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M. Kato, C.N. Kato / Inorganic Chemistry Communications 14 (2011) 982–985
(ε 6787 M−1cm−1), and 405 nm (ε 1346 M−1cm−1), as shown in Fig.
S4. The broad bands at 251 nm and 324 nm were assigned to the
charge transfer (CT) bands of W–O and a broad band at 405 nm due to
the two platinum(II) atoms, which was red-shifted relative to that of
cisplatin (λ=366 nm).
Stability of the two platinum(II) species of 1 with cis-configuration in
DMSO and aqueous media
It has been reported that cisplatin undergoes rapid ligand
exchange reactions and slow isomerization to the trans-isomer in
dimethylsulfoxide and water [1,11–14]. In addition, significant
inhibition of the isomerization of cisplatin in an aqueous solution by
addition of Cl− ions has been reported [12]. In our experiments, 1H
NMR spectra of cisplatin in DMSO-d6 showed that the cis-isomers,
including species formed by the ligand exchange reaction with DMSO,
were slowly isomerized to trans-isomers at 30 °C. The cis-isomers
remained after 12 h (77%) and 92 h (50%), as shown in Figs. 3 and S5.
The 31P NMR spectra of Me4N-1 (Fig. S6) in DMSO-d6 showed a signal
at −12.84 ppm after 12 h, indicating that polyoxoanion 1 was stable
after 12 h; however, a new signal at −12.96 ppm appeared after
23.5 h, and the intensity increased with time. The 1H NMR spectra
(Fig. S7) in DMSO-d6 showed two signals due to the cis-platinum(II)
moieties in 1 at 4.21 ppm and 4.36 ppm, and 95.1% of cis-platinum(II)
moieties remained after 12 h; thus, 1 was more stable than cisplatin
under the same conditions (Fig. 3). However, new signals at 4.26 ppm
and 4.43 ppm appeared in the 1H NMR spectra after 23.5 h, and the
intensities increased with time. No signals due to the cis- (3.97 ppm
and 4.65 ppm) and trans- (3.50 ppm and 4.39 ppm) platinum(II)
species eliminated from the vacant site of 1 were observed in 1H NMR
spectra. This was also supported by the result that no signal due to the
Keggin-type mono-lacunary polyoxotungstate (−10.12 ppm) was
observed in 31P NMR spectra. Thus, the new signals might be due to
the ligand exchange reaction of one or two ammonia molecules of
platinum(II) sites with DMSO [13]. For the 31P NMR spectra of 1 in
D2O, a signal at −12.66 ppm due to the central phosphorous atom in 1
was observed after one week at ca. 25 °C, suggesting that cis-platinum
(II) species in 1 were quite stable in aqueous solution with no
additives (Fig. S8).
Fig. 2. 183W NMR spectrum of 1 in DMSO-d6. The spectrum was referenced to an
external standard of saturated Na2WO4-D2O solution (substitution method).
K7[α-PW11O39]⋅13H2O (δ −10.12) and Na3[α-PW12O40]⋅12H2O [10]
(δ −14.67).
The 1H NMR spectrum of Me4N-1 in DMSO-d6 showed two signals
at 4.18 ppm and 4.32 ppm (Fig. S3). These signals were assigned to the
two NH3 ligands coordinated to each platinum(II) atom. No signals
due to cisplatin or transplatin were observed. These results suggested
that non-equivalent platinum sites were formed by the coordination
of [cis-Pt(NH3)2]2+ to the oxygen atoms in a vacant site of [α-
PW11O39]
7−, as shown in Fig. 1. The 183W NMR spectrum of Me4N-1 in
DMSO-d6 showed six signals at −101, −102, −107, −116, −142,
and −162 ppm with 2:2:1:2:2:2 intensities, resulting in Cs symmetry
(Fig. 2). A clear 195Pt NMR spectrum could not be obtained due to the
low solubility of Me4N-1 in DMSO-d6; however, two broad signals
were observed. These results were consistent with the 1H NMR
spectrum of Me4N-1 in DMSO-d6, as mentioned above.
The UV–vis spectrum of Me4N-1 in water showed three absorption
bands at 251 nm (extinction coefficient: ε 40120 M−1cm−1), 324 nm
100
80
60
40
20
0
Summary
A Keggin-type polyoxotungstate-coordinated diplatinum(II) com-
plex was synthesized. A tetramethylammonium salt, [(CH3)4N]3
[PW11O39{cis-Pt(NH3)2}2] (Me4N-1), was obtained as an analytically
pure, yellow powder in 35% yield. The characterization of compound
Me4N-1 was accomplished by elemental analysis, TG/DTA, FT-IR, UV–
vis, and (1H, 31P and 183W) NMR spectroscopy. The two cis-platinum
(II) moieties, [cis-Pt(NH3)2]2+, were coordinated each to two oxygen
atoms in a mono-vacant site of [α-PW11O39]
7− with Cs symmetry, and
the cis-configuration of platinum(II) moieties in 1 was highly stable in
DMSO and water with no additives.
Acknowledgements
This work was supported by a Hayashi Memorial Foundation for
Female Natural Scientists. The research was partially carried out using
an instrument at the Center for Instrumental Analysis of Shizuoka
University.
0
20
40
60
Time (h)
80
100
Fig. 3. Time course for the relative amount (%) of the cis-platinum(II) moiety in 1 (○)
and a cis-isomer that was the sum of cisplatin and the species formed by a ligand
exchange reaction with DMSO (△). The relative amount was calculated using the
integrated value of a signal at 4.36 ppm for 1 and (3.97 ppm and 4.65 ppm) for cisplatin.
Experimental conditions: Me4N-1 (ca. 20 mg) or cisplatin (ca. 10 mg) was dissolved in
3 mL of DMSO-d6 containing 0.021 mmol of DSS. These solutions were kept in a water
bath at 30 °C for 92 h. The time course of these solutions was monitored by 1H NMR
spectroscopy after 0.5, 2, 5, 12, 23.5, and 92 h.
Appendix A. Supplementary data
Supplementary data for this article can be found online at
doi:10.1016/j.inoche.2011.03.049.

M. Kato, C.N. Kato / Inorganic Chemistry Communications 14 (2011) 982–985
[6] K. Nomiya, H. Torii, C.N. Kato, Y. Sado, Chem. Lett. 32 (2003) 664.
985
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